1. Field of the Invention
This invention relates to magnetic printing processes and devices.
2. Description of the Prior Art
One form of copying process in wide usage is the electrostatic copying process. Operation of such a process may provide difficulties in that large black areas may not be amenable to copying and the document to be copied may have to be reimaged each time a copy is made. The overcoming of these difficulties may be economically prohibitive. It is well known that audio signals and digital data can be recorded on magnetic materials. Magnetic field configurations in the form of alphabetical characters and pictures can also be produced by selective magnetization or demagnetization of the surface of a ferromagnetic chromium dioxide film. The resultant fields are strong enough to attract and hold small magnetic particles such as iron powder. The development, that is, the making visible, of such a latent magnetic image can be effected by contacting the image surface with a magnetic developer, usually referred to as a magnetic toner, consisting of ferromagnetic particles and pigments encapsulated in a thermoplastic resin binder. Such a development process is commonly known as decoration of the latent magnetic image. The developed image can then be transferred to and fixed on paper, thus providing a black-on-white copy of the latent image. Operation of such magnetic processes, however, may not be completely free of difficulties. For example, since most magnetic toner particles are attracted by both electrostatic and magnetic fields, stray electrostatic charges which are present on the magnetic surface or toner particles may interfere with the interaction of the magnetic image and the magnetic toner particles. More specifically, a portion of the magnetic surface other than that containing the magnetic image may attract enough magnetic toner particles to render unsatisfactory the paper print which subsequently is produced.
There is extensive prior art in the fields of magnetic recording tapes and thermomagnetic recording. U.S. Pat. No. 3,476,595 discloses a magnetic recording tape which is coated with a thin layer of a cured complex of silica and a preformed organic polymer containing a plurality of alcoholic hydroxy groups. The disclosure includes coated, ferromagnetic, chromium dioxide, magnetic recording tapes. Discussions of acicular chromium dioxide and magnetic recording members bearing a layer of such material may also be found in U.S. Pat. Nos. 2,956,955 and 3,512,930. U.S. Pat. No. 3,554,798 discloses a magnetic recording member which is relatively transparent to light (transmits 5 to 95%) and which includes a plurality of discrete areas of hard magnetic particulate material supported thereon and bound thereto. A magnetically hard material is a material which is permanently magnetizable below the Curie point of the material, as opposed to a magnetically soft material which is substantially non-permanently magnetizable under similar conditions below the Curie point of the material. Chromium dioxide is disclosed as an example of a hard magnetic material. Decoration of the image may be effected by means of a magnetic pigment, for example, a dilute, alkyd-oil/water emulsion, carbon black-based printing ink. U.S. Pat. No. 3,522,090 is similar in disclosure to U.S. Pat. No. 3,554,798 in that it also discloses a light-transparent recording member. However, it also discloses that the magnetic material which is capable of magnetization to a hard magnetic state (on the recording member) may have a coating of a reflective material which is so disposed that the magnetic material is shielded from exposing radiation while the adjacent uncoated portion of the recording member transmits 10 to 90% of the exposing radiation. The reflective coating can be a metallic reflector, such as aluminum, or a diffuse reflective pigment, such as titanium dioxide. U.S. Pat. No. 3,555,556 discloses a direct thermomagnetic recording (TMR) process wherein the document to be copied is imaged by light which passes through the document. U.S. Pat. No. 3,555,557 discloses a reflex thermomagnetic recording process wherein the light passes through the recording member and reflects off of the document which is to be copied. Thus, in the direct process, the document must be transparent but the recording member need not be transparent, whereas in the reflex process, the recording member must be transparent but the document need not be transparent. For the recording member to be transparent, it must have regions which are free of magnetic particles, that is, a non-continuous magnetic surface must be used.
U.S. Pat. No. 3,627,682 discloses ferromagnetic toner particles, for developing magnetic images, that include binary mixtures of a magnetically hard material and a magnetically soft material, an encapsulating resin and, optionally, carbon black or black or colored dyes to provide a blacker or colored copy. "Nigrosine" SSB is disclosed as an example of a black dye. The encapsulating resin aids transfer of the decorated magnetic image to paper and can be heated, pressed or vapor softened to adhere or fix the magnetic particles to the surface fibers of the paper. Ferromagnetic toner particles of the type disclosed in U.S. Pat. No. 3,627,682 are disclosed as being useful in the dry thermomagnetic copying process of U.S. Pat. No. 3,698,005. The latter patent discloses such a dry thermomagnetic copying process wherein the magnetic recording member is coated with a polysilicic acid. The use of the polysilicic acid coating on the recording member is particularly useful when the magnetic material on the recording member comprises a plurality of discrete areas of particulate magnetic material because a greater number of clean copies can be produced. The polysilicic acid, which is relatively non-conductive, exhibits good non-stick properties. Thus, toner particles which are held to the surface of the recording member by nonmagnetic forces can be easily removed without removing the toner particles which are held to the surface of the recording member by magnetic forces. U.S. Pat. No. 2,826,634 discloses the use of iron or iron oxide magnetic particles, either alone or encapsulated in low-melting resins, for developing magnetic images. Such toners have been employed to develop magnetic images recorded on magnetic tapes, films, drums and printing plates.
Japanese 70/52044 discloses a method which comprises adhering iron particles bearing a photosensitive diazonium compound onto an electrophotographic material to form an image, transferring the image onto a support having a coupler which is able to form an azo dye by reaction with the diazonium compound, reacting the diazonium compound and the coupler and thereafter removing the iron particles. U.S. Pat. No. 3,530,794 discloses a magnetic printing arrangement wherein a thin, flexible master sheet having magnetizable, character-representing, mirror-reversed printing portions is employed in combination with a rotary printing cylinder. The master sheet, which consists of a thin, flexible non-magnetizable layer, such as paper, is placed on top of and in contact with a layer of iron oxide or ferrite which is adhesively attached to a base sheet. The combined layer and base sheet are imprinted, for example, by the impact of type faces, so that mirror-reversed, character representing portions of the iron oxide layer adhere to the non-magnetizable layer, thus forming magnetizable printing portions on same. Thereafter, the printing portions are magnetized and a magnetizable toner powder, such as iron powder, is applied to and adheres to the magnetized printing portions. The powder is then transferred from the printing portions to a copy sheet and permanently attached thereto, for example, by heating. U.S. Pat. No. 3,052,564 discloses a magnetic printing process employing a magnetic ink consisting of granules of iron coated with a colored or uncolored thermoplastic wax composition. The magnetic ink is employed in effecting the transfer of a printed record, using magnetic means, to paper. U.S. Pat. No. 3,735,416 discloses a magnetic printing process wherein characters or other data to be printed are formed on a magnetic recording surface by means of a recording head. A magnetic toner which is composed of resin-coated magnetic particles is employed to effect transfer of the characters or other data from the recording surface to a receiving sheet. U.S. Pat. No. 3,250,636 discloses a direct imaging process and apparatus wherein a uniform magnetic field is applied to a ferromagnetic imaging layer; the magnetized, ferromagnetic imaging layer is exposed to a pattern of heat conforming to the shape of the image to be reproduced, the heat being sufficient to raise the heat portions of the layer above the Curie point temperature of the ferromagnetic imaging layer so as to form a latent magnetic image on the imaging layer; the latent magnetic image is developed by depositing a finely divided magnetically attractable material on the surface of the ferromagnetic imaging layer; the imaging layer is uniformly heated above its Curie point temperature after the development to uniformly demagnetize it; and, finally, the loosely adhering magnetically attractable material is transferred from the imaging layer to a transfer layer.
German Pat. No. 2,452,530 discloses electrophotographic toners comprising a magnetic material coated with an organic substance containing a dye which vaporizes at 100.degree. to 220.degree. C, preferably 160.degree. to 200.degree. C, at atmospheric pressure. The magnetic material is preferably granular iron and/or iron oxide and the coating is a water-insoluble polymer melting at about 150.degree. C, e.g., polyamides, epoxy resins and cellulose ethers and esters. Both basic and disperse dyes can be used in the toners. The toners are from 1 to 10 microns in diameter and may also contain silicic acid as anti-static agent. Colored or black copies are formed by toner development of the latent image on a photo-conducting sheet of ZnO paper, followed by transfer of the dye in the vapor phase to a receiving sheet by application of heat and pressure.